In order to avoid cross infection caused by repeated use of the syringe, a disposable syringe is extensively used in medical practices. A typical disposable syringe comprises a barrel, a push rod reciprocatingly moveable in the barrel and a rubber piston in contact with the inner wall of the barrel. A distal end of the push rod and a needle seat mounted at a distal end of the barrel form snap-fitting mechanisms which can be engaged with each other. On the push rod is formed a stop flange located at a proximal side of the snap-fitting mechanism, and the rubber piston is mounted between the snap-fitting mechanism of the push rod and the stop flange. The stop flange is a boss-shaped structure and approximately rigid as compared with the rubber piston. During injection, the stop flange pushes and presses the rubber piston so that the rubber piston pushes a liquid medicament out of the syringe. When the rubber piston is pushed to a zero line, i.e., when the injection is finished, the rubber piston is braked. At this time, the push rod continues to be pushed and pressed, the stop flange will press the rubber piston into deformation so that the push rod can continue to advance distally such that the front end of the push rod is snap-fitted with the needle seat. Then, the push rod is pulled proximally, the needle seat is pulled in the barrel via the snap-fitting mechanism to complete self-destruction of the syringe and prevent the syringe from reuse.
However, in such disposable syringe in the prior art, in order to obtain an additional stroke whereby the distal end of the push rod continues to advance to engage with the needle seat upon completion of the injection, there is a need to apply a larger force to the push rod to cause the rubber piston to produce sufficient deformation. This causes the syringe strenuous in use and increases the patient's pains.